It depends on how your house is built and where you place the AccessPoint. It is different for every house. In addition, the quality of the WLan end devices also varies.
My suggestion: Buy one and try it out. If it's not enough, you can always buy a second one and keep trying.

No one can give you a general answer to that. In principle, the Ubiquitis are known for good range, but whether this works in your case depends on the nature of the building and a few other factors. To be really sure, it's best to run a LAN cable from the middle floor to the upper and lower floors and place an access point there. Unfortunately, it is still the case that a cable is many times more reliable than any WLAN.